In the manufacture of semiconducting devices, many processes are carried out in a furnace commonly referred to as a diffision furnace, although such furnaces are not limited to diffusion operations. A typical furnace consists of an openened elongate round or rectangular tube. An electrical heating element surrounds the tube, and may be formed with independently controlled sections to maintain a desired temperature profile in a region defined by the central portion of the tube. Such a tube will usually have a necked-down end to accept one or more input tubes carrying gases, the opposite end being open to a scavanger member for exhaust of the process gases.
In a typical process, a number of semiconductor wafers (e.g., thin slices of single crystal silicon) are placed in a carrier, called a boat, either in substantially vertical or substantially horizontal position and the boat is inserted into the central region of the furnace. Process gases are introduced in one end, pass over the wafer and exhaust out the other end. The hot reacting wafers are extremely sensitive to impurities; a few hundred parts per billion of certain elements, for example sodium, can "poison" a load of semiconductors so that they are useless. To minimize exposure of the wafers to such impurities, diffusion furnaces are typically lined with quartz.
To assure uniformity of composition of the semiconductor and therefore reproducibility of desired properties, it is essential that each wafer react identically to every other wafer in the furnace, and that each load of wafers reacts identically to every other load. Typically, this requires a region of isothermal temperature. Generally it is desired to maintain a central "flat" zone isothermal to within .+-.0.5.degree. C. or better. In some chemical processes, the rate of chemical reaction can vary as much as 1.5% per .degree.C. whereas variations in end product of more than .+-.5% are generally unacceptable in modern semiconductor devices. To achieve uniform reaction, standard furnaces have a helically wound resistance heating element surrounding the quartz tube that is divided into three sections by standoff connections welded into the element: a central heater surrounding the central region, and two independently controlled guard heaters, one at each end. In conventional operation the guard heaters are set at temperatures somewhat higher than the temperature set for the central heater, thereby compensating for heat loss through the open ends of the tube, and promoting an isothermal region in a portion of the central section of the tube. A furnace constructed in such a manner must have a large length to diameter ratio, e.g. about 10-15, in order to create a useable isothermal region. Such a furnace therefore requires not only large amounts of floor space but also is excessively energy intensive, because a substantial portion of the furnace is heated above the desired operating temperature of the isothermal region.
The present invention provides a furnace which can achieve an isothermal region with a tube having a low length to diameter ratio, without use of excessive amounts of energy. More particularly, in accordance with one aspect of the present invention, a pair of end walls are located at opposite ends of an elongate chamber in which there is a temperature controlled isothermal region. Means are provided for heating the temperature controlled region to the desired isothermal temperature and the end walls are independently heated by guard heater sections of the heating element such that their temperature is maintained at substantially the same temperature as the temperature controlled region. This results in the entire length of the temperature controlled region being isothermal. At least one end wall is movable to facilitate loading and unloading of the furnace.
Temperature control is achieved by placing a thermoelectric sensing device centrally of the temperature controlled region for feed-back control of the central heater section. The temperature of each end wall is controlled by two thermoelectric sensors, one disposed substantially centrally of the temperature controlled region, and one disposed in the proximal outer edge of the temperature controlled region, which may be substantially in the plane of the end wall. In operation, the temperature levels of the end walls are controlled so as to be maintained at the temperature of the central region, i.e., a zero "delta T" operation.
In accordance with another aspect of the invention, for some operations when a temperature gradient is desired, one end wall can be heated to a specified temperature above the central region, and the other end wall heated to the same temperature below the central region, i.e., commanding a positive delta T for one guard heater and a negative delta T for the other guard heater.
In a further embodiment, a combustor, i.e. a chemical reactor, can be located within one of the guard heater sections; in this case the wall on that end is located at the outer end of the guard heater. The present invention permits an exothermic reaction to take place in the combustor without destroying the desired temperature profile of the central region. This results from feed-back control of the end walls relative to the central heater; the sensor adjacent the combustor is heated by the combustor so that only that amount of electrical current is fed to the adjacent guard heater to bring that guard heater up to the desired temperature.
By using the present invention, the electrical consumption of a diffusion operation is estimated to decrease by 6 kilowatt hours per tube compared to a typical commercial operation. Additionally, the tube and associated equipment occupy a substantially smaller space, about 35% less than competitive equipment. Not only is there a savings in size and in direct energy requirements but because of the small size of the tube, smaller and therefore less expensive electrical power supplies and components can be used. Radiant heat losses are reduced thereby lowering electrical requirements for air conditioning and cooling water.